By Paul Knieste
A technique used to plot a basic course, dead reckoning has an ominous name but serves a useful purpose for boaters who take short distance trips. The technique is fairly simple, employing easy math and requiring only a nautical chart, a parallel ruler, and a pencil.
A nautical chart gives us a wealth of information including the location of buoys, channels, and the type of bottom. Additionally, a chart has an image of the compass rose, which shows the principal directions of north, south, east, and west. Some boaters may not be aware that there is a difference between magnetic north shown on a compass and true north on a chart — known as the magnetic variation. As the location of magnetic north changes from year to year, some calculations are necessary to account for this difference when plotting a course. That’s why it is essential to update paper and electronic charts annually, as the center of the compass rose shows the magnetic variation for the year the chart was printed.
Dead reckoning does not take into account such factors as tide, drift, current, and wind speed, but it is useful to estimate how to get from where you are to where you want to go if it’s not too far away. The process begins by finding the points where you want to depart and where you desire to reach. Then you employ the chart, the parallel ruler, and pencil, as you’ve learned from a boating safety class or a video tutorial such as the ones featured after this article. My tip: remembering the saying, “East is least (subtract) and west is best (add),” is a handy aide as you calculate.
Once you calculate and set the compass heading, it’s difficult to stay on the precise heading due to wind and wave action, or the glare of the sun. A five-degree error right or left of your course is usually acceptable when you’re navigating by dead reckoning.
Since most boaters rely heavily on electronic navigation devices, having a chart and knowing the dead reckoning technique is invaluable in the event your electronics aren’t operating. If they are and you’ve still employed this age-old mariner’s skill, you may notice your GPS objects to the heading, but it will not usually be by much.
A final note: Magnetic deviation is different from magnetic variation, but it can also affect course. Metal objects and electrical current can disrupt compass readings, but unless you pilot a steel-hulled boat, you can mitigate this deviation by installing your compass away from electrical devices (especially speakers that have magnets). A simple way to check for deviation is to turn your electronics on and off to see if your compass card moves.
Sea-More Dead reckoning videos
